The present invention relates to methods for decontaminating concrete and, more specifically, to methods used for in-situ thermal desorption of contaminants from concrete.
Because many concrete building surfaces have been contaminated with PCBs since their industrial use became prominent in the 1940's as a dielectric insulating oil and heat transfer fluid, it has been desirable to provide an apparatus and a method for decontaminating concrete which contains volatile or semi-volatile materials, such as PCBs. Present technology is not available to thermally desorb contaminants from concrete in a nondestructive manner; however, technology has been described which can thermally desorb contaminants from soil at reduced pressure.
The degree of contamination of concrete is usually determined as a surface concentration. The standard method for quantifying the contamination is by wipe tests in which solvent-soaked pads are wiped across a given area of the surface and thereby soak up PCBs that dissolve in the solvent. The PCBs are then extracted from the pad to determined a contamination level in mass per unit area. These wipe tests are also used to determine the residual PCBs left after a remedial treatment. Present regulations usually require cleanup levels of less than 10 .mu.g/100 cm.sup.2. However such wipe tests cannot ensure that PCBs in the subsurface region will not diffuse or permeate back to the surface with time. In addition, if a concrete structure is going to be destroyed and deposited in a landfill, the material will be subject to regulations which are based on bulk concentrations or mass of PCBs per unit mass of material. For these reasons it is desirable to develop a cost effective technique for removing PCBs from the subsurface as well as the surface of concrete.
The current technologies available for remediation of concrete contaminated with semi-volatile organics such as PCBs can be divided into two categories: surface removal methods and chemical methods. In surface removal methods the exposed layer of contaminated concrete is removed by any of several technologies. These include scarifying, scabbling, spalling induced by mechanical or thermal stresses, sand blasting, liquid jet, frozen carbon dioxide blasting, and controlled explosion. The advantage of surface removal methods is that if the contamination is confined to the surface layer, the technique is certain to remove the contamination regardless of contaminant type. The obvious drawbacks of such methods are that a large volume of contaminated waste is generated which must be processed further or stored in a regulated hazardous waste site and the new concrete surface must be refinished for future use. Typical costs for surface removal techniques range from $50 to $250/m.sup.2. The volume of concrete rubble produced from such methods is about 10 L/m.sup.2.
Chemical methods involve applying a liquid, foam or paste containing chemicals which either destroy the contaminants within the concrete or remove them via dissolution and mobilization. The disadvantages of these methods are that large volumes of liquid hazardous waste are generated, the dissolved contaminants may migrate deeper into the concrete, and it is difficult to control the depth to which the decontamination occurs. In some cases the concrete surface is degraded. The costs for wet methods of cleaning range from $30 to $200/m.sup.2.
Both in-situ volatilization of organics from soil and ex-situ volatilization from soil and sludge are widely used for remediation, especially where contamination consists of VOCs (Volatile Organic Compounds) or CVOCs (Chlorinated Volatile Organic Compounds). As a guide to appropriate volatilization methods, the following rules of thumb based on contaminant vapor pressure at ambient temperatures may be useful:
______________________________________ Vapor Pressure at Ambient Temperature, mb Volatilization Methods ______________________________________ 5 Natural Convection 1 Forced Convection 0.1 Heating to 120-250.degree. C. 0.01 Heating to 250-550.degree. C. ______________________________________
In-situ soil decontamination processes generally employ forced convection at ambient or relatively low temperatures. The convection can be generated by either pressurization, suction or a combination and elevated temperatures are achieved by pre-heating pressurized air or injecting steam into the soil. A process developed by Drexel University is unique among similar processes in utilizing an impermeable mat over the soil to capture contaminants. Ex-situ processes carried out in batch or continuous equipment are capable of reaching higher temperatures and lower pressures than in-situ processes; these conditions can either improve VOC/CVOC recovery efficiency or allow faster removal rates of less volatile contaminants. While ex-situ processes normally address soil or sludge contamination, volatilization from construction debris such as concrete is clearly possible.
Many continuous ex-situ remediation processes resemble rotary kilns which not only operate at temperatures sufficient to volatilize organic contaminants but also attain conditions which oxidize contaminant vapors to harmless products. Temperatures required for oxidation are typically in the range of 875.degree. to 1375.degree. C. Such temperature would destroy PCB vapors, but destruction by oxidation would require that the process be permitted for incineration. Thus, any thermal desorption process for concrete should operate without exceeding an equipment temperature of about 450.degree. C. at any point, thereby avoiding PCB oxidation and the required permitting.
Two specific volatilization processes for decontaminating concrete (or other noncombustible solids) and soil, respectively, are related to the present application. The first is flaming, in which an open flame is directed against building surfaces such as walls. As with the present invention, flaming is suited to subsurface decontamination of porous materials by volatilization. Achieving a temperature of 300.degree. C. at a depth of 5 cm requires 16 minutes for concrete and 25 minutes for brick has been reported. The process was applied at the Frankford Arsenal to structures contaminated with explosives. In that instance the decomposition and oxidation of volatilized contaminants by the open flame was considered to be an advantage. But applying a similar decontamination process to structures contaminated with PCBs is unlikely from both safety and regulatory standpoints. In particular, the off-gases would be very difficult to control.
The second related process, developed by the Shell Oil Company, is similar to the Drexel process in using an impermeable mat or sheet to collect contaminants volatilized at reduced pressure from heated soil. But the soil is heated from the surface by a flat electrical resistance heater which is located under the sheet and which can reach temperatures as high as 1000.degree. C. As the subsurface soil is heated, organic contaminants are vaporized as in the process we have proposed for concrete. But the permeability of sandy or silty soils is from 3 to 6 orders of magnitude greater than that of concrete, and they contain several times more free or loosely bound water than concrete. As a result the "vacuum" collection system drawing contaminants from the underside of the sheet actually collects large amounts of air drawn through the surrounding soil and water vapor volatilized from the heated soil. This air constitutes a steady state forced convective flow under the applied pressure difference, as opposed to the transient convective flow of background vapor in volatilization from concrete at reduced pressure. Both air flow and water vaporization during soil heating can require a substantial energy input as compared to heating concrete. The high surface temperature of the Shell process is necessary to achieve heat fluxes that will raise subsurface soil to the desired temperatures in reasonable times; however, in concrete where contamination is usually within 1-2 inches of the surface, such high temperatures are not essential.
Thermal desorption of contaminants from solid materials is a process that can be applied to volatile or semi-volatile contaminants. By heating the contaminated material at reduced pressures the volatile and semi-volatile species are vaporized and drawn out of the solid matrix.
Thermal desorption has been used extensively to clean excavated soils. The idea has been applied by Shell Oil Company to removing pesticides from soils in-situ by applying the previously mentioned heating blanket on the surface of the soils and drawing a partial vacuum underneath the blanket. In the Shell process, the soil surface was heated as high as 1000.degree. C. and the pesticides were destroyed by high temperature oxidation.
While there are many similarities between the in-situ soil remediation and the apparatus and method of the present invention for concrete decontamination, there are some key differences. For example, the much lower hydraulic permeability of concrete decreases the significance of air and vapor flow to purge the contaminants from the solid matrix. In concrete, the transport of contaminants out of the matrix will be much more dependent on diffusion and vaporization of hydrated water. Because of the lower air flow rates a greater vacuum pressure should be achievable over concrete thereby reducing the temperature required to volatilize the contaminants.
In the case of PCB decontamination inside a building, it may be desirable to minimize the temperature of the heating elements to prevent the formation of toxic oxidation products such as chlorinated dibenzofurans. Also, to preserve the structural integrity of concrete, the temperatures to which the concrete surface is heated should be kept as low as possible while still removing the contaminants.
In U.S. Pat. No. 4,670,634, Vinegar et al. (Shell patent) propose a method for in-situ decontamination of soil designed to thermally desorb contaminants from soils at reduced pressure. In their embodiment, an impermeable sheet covers permeable insulating material which in turn covers electrical resistance heating elements. These heating elements are in direct thermal contact with the contaminated soil.
One of the problems with the Shell Patent, when applied to PCBs, is that the heater elements will be significantly hotter than the surface of the substrate (soil as described in the patent) which is being heated. These hot heater surfaces may cause the destruction of any PCB vapors which come in contact with them. Such destruction may be undesirable if regulatory agencies require PCB incinerator permitting. Another problem with direct contact of the heating elements with the substrate is that there will be hot spots or regions of steep temperature gradients (uneven heating) in the vicinity of contact points between the heater and the substrate. This is a disadvantage when using the Shell apparatus and method on concrete, because the resulting thermal stresses may weaken or crack the concrete. In addition, the placement of an impermeable layer over a permeable insulator will cause the insulation to become contaminated. This may present difficulties when moving the equipment to another site.
What is desired, therefore, is an apparatus and method for effectively removing volatile or semi-volatile, such as PCBs contamination from concrete which avoids the destruction of large quantities of concrete which avoids the need to deposit destroyed cement in a hazardous landfill; which can be utilized at depths greater than prior chemical methods; which can remove contaminants to an acceptable level while minimizing the hazardous waste generated; which can reduce the level of contamination down to low levels; and which is simple and inexpensive to operate and decontaminate.